The present invention relates to an air-fuel ratio detector and method for detecting an air fuel ratio and, more particularly, to an air-fuel ratio dectector and method for detecting an air fuel ratio of an air fuel mixture supplied to an internal combustion engine.
Air-fuel ratio detectors or oxygen concentration sensors have been proposed wherein electrodes are provided on either side of a bottom of a tubular solid electrolyte formed of, for example, zirconia, with the atmosphere being introduced into an interior of the tubular solid electrolyte, and with an outer side of the tubular solid electrolyte being exposed to the gas to be measured.
In an air-fuel ratio detector of the aforementioned type, an output is produced such that the electromotive force is incrementally changed at a theoretically ideal or optimum air-fuel ratio of, for example, 14.7. For this reason, a detector of the aforementioned type is generally widely employed in control of internal combustion engines for motor vehicles to determine whether the air-fuel mixture supplied to the internal combustion engine is lean or rich relative to the theoretically ideal or optimum air-fuel ratio.
Recently, an air-fuel ratio detector has also been developed for detecting a lean air-fuel ratio so as to burn a lean mixture thereby conserving fuel. For example, in FIGS. 4 and 5 of U.S. Pat. No. 4,282,080, corresponding to Japanese Laid Open Patent Application No. 55-125548, a detector is proposed which comprises a solid electrolyte and a porous diffusion resistor, with a threshhold current being measured so as to detect the lean air-fuel ratio.
Additionally, in FIG. 1A of, for example, U.S. Pat. No. 4,158,166, corresponding to Japanese Patent Laid-Open Application No. 53-66292, a further detector is proposed which includes a solid electrolyte and a single-holed diffusion resistor. In this proposed detector, oxygen is pumped into a reference chamber by a solid electrolyte pump and reacts therein with CO flowing into the reference chamber through the single hole thereby detecting a rich air-fuel ratio.
Furthermore, in U.S. Pat. No. 4,304,652, corresponding to Japanese Laid-Open Patent Application No. 55-166039, a detector is proposed which includes a solid electrolyte and a porous diffusion resistor, with a direction of the current passing the solid electrolyte being selectively reversed so as to determine whether the air-fuel ratio is lean or rich.
In, for example, U.S. Pat. No. 4,272,331, a detector is proposed which includes a pump cell which adds or removes gaseous oxygen from a volume, with a sensor cell being provided for detecting an EMF developed as a result of the pumping action of the pump cell. An external circuit causes a pump current to flow which removes the oxygen from the volume until the EMF reaches a reference voltage. Then, pumping is reversed until the EMF reaches another reference voltage, with the pumping pattern being caused to repeat thereby establishing an oscillatory period that is proportional to the partial pressure of the oxygen.
While each of the above described detectors function in particular ranges, none of the detectors can provide a complete detection over a wide range of the air fuel ratios from a rich to a lean operation.
Accordingly, the aim underlying the present invention essentially resides in providing an air-fuel ratio detector which is capable of detecting a wide range of air-fuel ratios from rich to lean.
In accordance with advantageous features of the present invention, an air-fuel ratio detector is provided which includes an oxygen ion conductive solid electrolyte, first and second electrodes respectively provided on each side of the electrolyte, a diffusion resistor provided on the first electrode and exposed to the measured gas, as well as a means for supplying a current between a first electrode and the second electrode so as to feed oxygen from the second electrode to the first electrode through the solid electrolyte during a predetermined time, and then withdraw oxygen from the first electrode to the second electrode through the solid electrolyte. Additonally, means are provided for measuring the mobility or migration of the oxygen withdrawn from the first electrode to the second electrode through the solid electrolyte.
The mobility of migration of oxygen is detected on the basis of a variation rate of withdrawal current, an average value of the withdrawal current, or a period from the starting time of the withdrawal to the time when oxygen content near the first electrode becomes substantially zero.
In accordance with the present invention, oxygen is intially fed from the second electrode to the first electrode and the partial pressure of the oxygen near the first electrode provides a value which is proportional to the oxygen partial pressure in the measured gas in a lean air-fuel ratio and, in an inverse proportion to the partial pressure of carbon monoxide in the measured gas in a rich air-fuel ratio; therefore, it becomes possible to measure air-fuel ratios ranging from rich to lean.
In accordance with advantageous features of the present invention, the mobility of the oxygen, i.e., the migration of the oxygen ions, is detected in dependence upon a variation rate of current which passes between the second electrode and the first electrode at a beginning of the oxygen withdrawal.
It is also possible in accordance with the present invention to detect the mobility or migration of the oxygen ions based upon an average value of a current which passes between the second electrode and the first electrode during the oxygen withdrawal.
Advantageously, in accordance with further features of the present invention, the mobility of the oxygen, i.e., migration of the oxygen ions, may be detected in dependence upon a period from the starting time of the oxygen withdrawal to a time when the oxygen content near the first electrode reaches a predetermined value which is substantially equal to zero.
Advantageously, it is possible to detect when the oxygen content near the first electrode has become a predetermined value in dependence upon a change in the electromotive force produced between the first electrode and the second electrode.
The second electrode may be exposed to the atmosphere or, alternatively be exposed to the gas to be measured.
In accordance with still further features of the present invention, a diffusion resistor provided on the first electrode and exposed to the measured gas is fashioned as a porous diffusion resistor and, advantageously, the porous diffusion resistor may be covered with a porous protective layer which has a porosity greater than that of the porous diffusion resistor.
In accordance with additional features of the present invention, the diffusion resistor may include at least one cover which defines a chamber and includes an orifice and the solid electrolyte may be porous and serve also as the diffusion resistor.
In accordance with the method of the present invention a current is supplied between the first and second electrodes so as to feed oxygen from the second electrode to the first electrode and enable a withdrawing of oxygen from the first electrode to the second electrode through the solid electrolyte, and measuring a mobility of oxygen ions withdrawn from the first electrode to the second electrode through the solid electrolyte for determining the air fuel ratio.
In accordance with the method of the invention the step of measuring may include detecting a variation rate of current passing between the second electrode and first electrode at a beginning of the oxygen withdrawal.
It is also possible in accordance with the present invention to detect an average value of current passing between the second and first electrode during the oxygen withdrawal.
In accordance with yet further features of the method of the present invention, the measuring step may include detecting a period of time from a starting time of the oxygen withdrawal to a time when the oxygen content near the first electrode has reached a predetermined value substantially equal to zero, with the period of time being detected by, for example, a determining of a change in electromotive force provided between the first and second electrodes.
Accordingly, it is an object of the present invention to provide an air-fuel ratio detector and method of detecting an air fuel ratio which avoids by simple means, shortcomings and disadvantages encountered in the prior art.
Another object of the present invention resides in providing an air-fuel ratio detector which is simple in construction and therefore relatively inexpensive to manufacture.
Another object of the present invention resides in providing an air-fuel ratio detector which provides a complete detection over a wide range of air fuel ratios from rich to lean.
These and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which show, for the purposes of illustration only, several embodiments in accordance with the present invention.